Gas chromatography is an analytical technique which entails the separation and often identification of individual compounds, or groups of compounds, within a mixture. A gas chromatograph takes a small sample of liquid or gas (typically about 0.1 cubic centimeter), and identifies the amounts of various compounds within the sample, often in the form of a chromatograph. A chromatograph is a line chart with the horizontal axis identifying different compounds and the vertical axis giving the concentration. The total amount of a compound in a given sample is usually related to the area under the peak associated with that particular compound. No other analytical technique is as powerful and as generally applicable as is gas chromatography. It is widely used in most sectors of chemistry, biology, forensics, environmental studies, and many areas of research.
A gas chromatograph is typically composed of three major subsystems: an injection chamber, a column separator, and a gas detector. Each of these subsystems usually has an independent means of temperature control. In order to analyze a sample, it is first injected into the injection chamber where a continual flow or pressure of a carrier gas (hydrogen, helium, nitrogen, air, etc.) is maintained. The injection chamber is usually maintained at a temperature such that various compounds within the sample are vaporized and enter the separation column.
The separation column is a long glass or metal tube which is coated on its interior surface by an inert compound designed to impede the flow of different compounds by different amounts. The separation column is typically about 1 to 30 meters long and has an inner diameter of about 50 microns (μm) to 1 millimeter (mm). Even smaller columns have been fabricated in silicon substrates. The coating is referred to as the packing material and is one of the most important considerations when picking the desired column to analyze a particular sample. The carrier gas carries the evaporated compounds through the column. Different molecules diffuse through the column at different rates even though their stochastic differences may be small. Detailed analysis of compounds often involves the use of several columns with different packing materials.
Many different detection techniques are applied at the exit of the column to help create the desired chromatograph. Most importantly, the detector must be able to distinguish relative changes with respect to time of any physical property of the gas exiting the column. It is not necessarily important for the detector to identify the compounds exiting the column, but instead to be very sensitive to changes in composition of the exiting gas. Each detection system ideally leads to a chromatograph that may have particular advantages over other detectors for specific compounds. Some of the many types of detectors that are common include flame ionization detectors (FID), flame photometry detectors (FPD), nitrogen phosphorous detectors (NPD), electron capture detectors (ECD), thermal conductivity detectors (TCD), atomic emission detectors (AED), photoionization detectors (PID), electrical conductivity detectors (ELCD), mass spectrometer detectors (MS), discharge ionization detectors (DID), and chemiluminescence detectors.
Some detectors observe properties that can be measured without altering or destroying the gas being detected, such as thermal conductivity detectors. Most detectors, however, require external energy to excite or ionize the gas species, such as all flame-based detectors, ionization detectors and mass spectrometer detectors. These detection techniques often alter the compounds.
Each type of detector has its own advantages and disadvantages. They compete with each other primarily in their sensitivity to given classes of compounds, but also in dynamic range, linearity, universality, portability, and cost. Often compromises among these categories have to be made for specific applications.
A means of converting the observation into an electrical signal is a property of all detectors. Voltage or current is ultimately measured as a function of time and the result displayed on a printout or computer monitor. These results are based on the initial time where the sample was injected into the gas chromatograph. The time between injection and each peak is specific to a particular compound or group of compounds. The instrument is calibrated by injecting a single known compound and measuring the time between the injection and the corresponding peak on the chromatograph. This is repeated for all compounds of interest generating a table of delay times, often referred to as the retention time. The retention time for any compound will ideally be the same even if the compound is contained in a mixture of other compounds. However, these times vary for different columns. Injector chamber and column heating cycles also change the retention times.
Existing gas detectors suffer from several disadvantages. In particular, they are relatively large and consume large amounts of power. Accordingly, there is a demand for a gas detector that is smaller and uses less power.